1. Field of the Invention
The present invention relates to a honeycomb structure in which a plurality of honeycomb segments are integrally joined by a joining material, and more particularly, it relates to a honeycomb structure having a joining material layer in which a stress relaxing function and a joining strength are equal to or higher than those of a joining material layer of a conventional honeycomb structure, and having an excellent resistance to heat shock.
2. Description of Related Art
A honeycomb structure is broadly used as a collecting filter of a microscopic particulate matter, for example, as a diesel particulate filter (DPF) to collect and remove particulates included in an exhaust gas from a diesel engine or the like.
Such a honeycomb structure has, for example, a configuration in which a plurality of cells divided and formed by porous partition walls made of silicon carbide (SiC) or the like to become through channels of a fluid are arranged in parallel with one another in a central axis direction. Moreover, ends of the adjacent cells are alternately plugged (in a checkered pattern-like manner). That is, one end of one cell is open, and the other end of the one cell is plugged. Moreover, one end of the other cell adjacent to the one cell is plugged, and the other end of the other cell is open. A plurality of configurations in which such cells are arranged in parallel with one another in the central axis direction (hereinafter referred to as “the honeycomb segments”) are integrally joined to one another by a joining material to form the honeycomb structure.
According to such a structure, the exhaust gas can be purified as follows. First, the exhaust gas is allowed to flow into a predetermined cell (the inflow cell) through one end of the honeycomb structure. In this case, the other end of this cell is plugged, and hence the exhaust gas passes through a porous partition wall and is introduced into the adjacent cell (the outflow cell). Moreover, when the exhaust gas passes through the partition walls, the particulates in the exhaust gas are collected by the partition walls. Therefore, the exhaust gas purified through the outflow cells adjacent to the inflow cells is discharged.
To use such a honeycomb structure (the filter) continuously for a long period of time, the filter needs to be periodically subjected to a regeneration treatment. That is, when a pressure loss increased by the particulates deposited in the filter with an elapse of time is decreased to return a filter performance to an initial state, it is necessary to burn and remove the particulates deposited in the filter. At this filter regeneration, a large heat stress is generated by heating, thereby resulting in the problem that this heat stress causes defects such as cracks or damages in the honeycomb structure.
Therefore, to meet a request for enhancement of a resistance to a heat shock such as this heat stress, there is suggested a honeycomb structure of a divided structure in which a plurality of honeycomb segments are integrally joined via a joining material layer to impart a function of distributing and relaxing the heat stress (e.g., see Patent Documents 1 to 4). Moreover, according to such a honeycomb structure, the resistance to the heat shock can be improved to a certain degree.
Furthermore, in the honeycomb structure in which the plurality of honeycomb segments are joined by the joining material, it is suggested that a porosity distribution of this joining material layer portion is controlled to a suitable value, to impart the function of distributing and relaxing the heat stress, thereby obtaining the honeycomb structure having an excellent resistance to heat shock (see Patent Document 5). In this method, the joining material layer contains inorganic fibers, but the inorganic fibers include a grain-like inorganic substance called a shot which is a byproduct during manufacturing of the inorganic fibers. Due to the presence of this shot, the stress relaxing function and a joining strength deteriorate.
Moreover, to join the honeycomb segments, an SiC joining material is suggested in which in addition to SiC particles, acicular crystal particles or strip-like crystal particles made of an inorganic material are contained to enhance dispersibility and the joining strength (see Patent Document 6). However, in this method, the joining material layer becomes dense and has an increased Young's modulus, and hence there is the problem that the stress relaxing function against thermal expansion of the honeycomb segments deteriorates.    [Patent Document 1] JP-B-4392984    [Patent Document 2] JP-B-3889194    [Patent Document 3] JP-B-4592695    [Patent Document 4] WO 2006/103786    [Patent Document 5] WO 2007/111280    [Patent Document 6] JP-A-2008-7683